Author: Ida Lucy Iacobucci, 2015
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Chapter 2
FORMS OF RETINAL CORRESPONDENCE
NRC – Normal Retinal Correspondence
(Corresponding Retinal Elements)
ARC – Anomalous Retinal Correspondence
(Noncorresponding Retinal Elements)
I.
INTRODUCTION – HOROPTER & TYPES OF ARC
A. Physiological Factors2
Localization is based on physiological factors. Objective aspect is the
correct geometric localization of an object in space. The objective
line of direction (visual axis) is the line drawn from the fixation object
through space and the optical system of the eye to the retina.
Normally, light from an object located directly ahead stimulates the
fovea.
A retinal element refers to a rod or cone together with all the chain of
cells connecting that sensory cell with the occipital visual cortex.
Subjective aspect (projection) is the place in space where an object
appears to be situated which depends on the spatial value of the
particular retinal element stimulated. Each retinal element has its own
particular direction in which it localizes objects that stimulate it,
which is the visual direction of that retinal element. The fovea
normally has a zero value visual direction.
The objective line of direction visual axis and the subjective visual
direction normally coincide.
For every point on the retina of one eye there is a point on the retina
of the other eye which is stimulated by the same object. Retinal
elements in each eye, which normally are stimulated by the same
object, are called normal retinal corresponding elements. These are
corresponding retinal elements that have the same visual direction.
The foveas are corresponding points “par excellence”.
An object stimulating non-corresponding retinal points is seen
double because each point has a different visual direction. Slightly
disparate retinal elements are fused if the retinal elements are within
Panum’s fusional area and result in stereopsis instead of diplopia. The
amount of disparity fused is very small near the fovea but increases in
size as the peripheral retinal areas are stimulated.
B. Horopter
Locus of points in a space which at a given distance simultaneously
stimulate corresponding retinal elements therefore giving rise to a
single image.
Empiric horopter forms a variable line depending on the distance of
fixation object from the eye:
1. Fixation object 2 meters away - horopter is a straight line passing
through the fixation point.
2. Fixation object nearer than 2 meters – horopter is curved,
concave to the face.
3. Fixation object is beyond 2 meters – horopter is curved, concave
to the face.
Images falling on non-corresponding points (each having different
visual direction) are seen double. However, slightly disparate retinal
points to be stimulated by similar images produce a single image
because the retina points fall within Panum’s area (stereopsis).
Representative of Panum’s fusional space.
Fixation distance 1 METER –
horizontal meridian for the horopter (YY) one meter away.
{Y Y}
Horizontal meridian for the Horopter 1 meter away
{XX1, XX} Limits of Panum’s fusional areas in the retina in relation
to the horopter
A- The fixation point.
B- On the horopter - imaged on corresponding retinal points seen
as one.
C- Is off the horopter but within Panum’s fusional areas - imaged
on non-corresponding points but is seen with depth effect
(stereopsis) and not double.
D- Is off the horopter and outside Panum’s fusional area – imaged
on non-corresponding points seen double (physiologic diplopia).
Panum found that in the region near the fovea only a slight degree of
disparity was possible before stereopsis failed and diplopia occurs;
nearer to the periphery, more widely disparate retinal points give rise
to stereopsis rather than diplopia.
Therefore, imaging areas of disparate points are very small near the
fovea and increase in size as they approach the periphery of the
retina.
Panum’s area – Fusional area around corresponding points in the
retina.
Panum’s space: The space on either side of the horopter containing
objects in space, images of which will fall on slightly disparate retinal
elements within Panum’s fusional areas of the two retinas so that they
are found as stereopsis.
Anomalous retinal correspondence is an attempt of the strabismic
patient to obtain some semblance of normal binocular vision. It is a
binocular phenomenon though monocularly either eye picks up
fixations with the fovea.
In heterotropic patients non-corresponding retinal elements outside
of Panum’s fusional area are stimulated. Normal localization of the
deviated eye results in diplopia rather than fusion. Anomalous
localization allows an ‘extra foveal’ element of the deviating eye to
acquire a correspondence with the fovea of the fixing eye. In
heterotropia-anomalous, retinal correspondence may develop for the
purpose of eliminating diplopia (and confusion) and regaining a
semblance of binocularity with some form of stereopsis. It is a
binocular phenomenon, though monocularly either eye picks up
fixation with the fovea.
C. Two types of ARC
Harmonious ARC
30 Δ Esotropia
Unharmonious ARC
30 Δ Esotropia – Objective Angle
20 Δ Esotropia – Subjective Angle
Angle of anomaly is equal
to the angle of strabismus
Angle of anomaly is not equal to
the angle of strabismus
1. Harmonious ARC
Angle of anomaly is equal to the angle of strabismus and therefore
the subjective angle is zero. In harmonious ARC a retinal element in
the deviated eye which is at the exact point (A) where the image falls or
the deviating eye acquires correspondence with the fovea of the fixing
eye.
2. Unharmonious ARC
The subjective angle is not equal to the objective angle of strabismus.
The subjective angle is greater than zero but less than the objective
angle of strabismus. The fovea of the fixing eye acquires a
correspondence with external stimuli of the deviated eye but not at
the exact objective angle of deviation (as in harmonious ARC). The
pseudo fovea is at a point (B) less than the objective angle of
deviation. The patient with unharmonious ARC has learned to use
their eyes at a less crossed angle in an attempt toward binocularity.
NOTE: Some do not believe unharmonious ARC exists but it is
demonstrated on a variety of tests. It has been explained that it could
be due to the patient having a change in angle of strabismus and the
sensory adjustment hasn’t had time to ‘catch up’ with the change in
angle of strabismus. Bagolini has never been able to demonstrate
unharmonious ARC with the striated lens following surgical
correction and this definitely demonstrates a rapid sensory
adjustment. He feels that it exists because of the methods used in
testing.
D. Terminology
Clinically, we speak of the objective angle as the amount of deviation
measured by the examiner and neutralized so that the image of the
fixation object falls on the fovea of each eye, such as the alternate
cover and prism test or alternate cover test on the amblyoscope.
Subjective angle is the amount of correction in prism diopters in which
the patient informs the examiner that he can superimpose and fuse
the images of the test object.
Angle of anomaly is the difference of the objective angle minus the
subjective angle. In normal retinal correspondence the objective
angle and subjective angle are the same, Therefore, the angle of
anomaly is zero. In unharmonious ARC, the objective angle does not
equal the subjective angle. The subjective angle is less than the
objective angle; the angle of anomaly is not zero. In harmonious
ARC, the subjective angle is equal to the objective angle.
In summary:
Objective test – depends upon findings of the examiner – independent
of patient’s response.
Subjective test – depends upon a patient’s response.
Subjective angle – The angle on the synoptophore at which the patient
can superimpose test object slides is different from the objective
angle which shows ARC (at least 8Δ or 4o difference to demonstrate
ARC).
Objective angle – The angle to which examiner manipulates the arms of
the synoptophore so that the image of test object slides fall on the
fovea of each eye.
E. Definitions
Anomalous retinal correspondence can be defined in more than one
way.
1. ARC is present when two foveae no longer have a common
visual direction.
2. ARC is a binocular condition where the fovea of the fixing eye
is used in conjunction with a point other than the fovea of the
deviating eye.
3. ARC is a binocular condition where the angles of deviation
measured objectively and subjectively are not coincident. The
difference between the two angles is known as the angle of
anomaly.
4. ARC is a binocular condition where there has been a
rearrangement of the normally common visual directions of
the retinal elements of the two eyes. Corresponding retinal
elements lose their common visual direction while disparate
retinal elements acquire a common visual direction.
F. Prognosis for ARC to Develop
Anomalous retinal correspondence is more likely to develop in small
to moderate degree manifest deviations with a relatively constant
long-standing and stable angle. In large angles of deviation, there is
little reward for development of ARC because the peripheral retina
has very low visual attention. It is seldom present in accommodative
strabismus, intermittent strabismus, and incomitant deviations due to
variability of the angle of deviation. It is rare in alternating
strabismus.
Cases in which ARC is more likely to develop:
1. Moderate degree of ET (esotropia) and relatively stable angle
of deviation.
2. In small angles of deviation under 10 prism diopters the image
may fall within Panum’s fusional area.
II. SENSORY TESTS FOR ARC
Clinical tests are based on two principles: (1) to assess visual direction
of the two foveae, and (2) to compare the subjective localization of
images with the objective angle of deviation.
A. Diplopia Test - Method I
1. Red filter over preferred eye. Patient fixes on distant or near
light.
2. Place vertical prism (10Δ BU or BD) in front of one eye to help
overcome suppression.
Results: If red light is seen above white light in presence of 30Δ ET,
this indicates harmonious ARC.
B. Diplopia Test - Method II
1. Objective angle is neutralized with appropriate prisms and
checked by cover test.
2. Red filter is placed in front of one eye. Check again if
deviation is still neutralized, then ask patient if images are
superimposed. If images are not seen superimposed, reduce
prisms until the images are seen superimposed or change from
crossed to uncrossed or vice versa. If more than 8Δ difference
from objective angle, ARC is present.
C. Bagolini Striated Lens
Plano lenses have very fine striations etched in them. They are
placed in a trial frame; one is set at 45° and the other at 135° so that
the line images are seen obliquely as the patient fixes a light. A
person with single binocular vision will see a cross passing through
the light. ET with NRC (fixation light above), XT with NRC
(fixation light below).
If a person with an ET claims a cross at the light then they must
have ARC. If only one line is seen passing through the light,
suppression of one eye is indicated.
NOTE: The Bagolini lens has the same effect as the Maddox Rod
but does not dissociate the eyes by presenting an obstacle to vision
and it reduces the artificiality of the testing situation. It represents
the most ‘normal’ seeing conditions.
D. Bielschowsky After Image Test
One eye is occluded and the patient is instructed to fix a dot in the
center of a horizontal filament of light for approximately 20 seconds.
The light is then extinguished, rotated to 90°, the other eye occluded,
and the process is repeated. Present the horizontal light first to the
normally dominant eye and then the vertical light to the nondominate eye.
Results: The patient is instructed to close their eyes and observe the
white lines against his eyelids and draw the arrangement of lights seen
(the positive after image). Then the patient looks at a blank wall,
blinks their eyes, and observes the position of the black lines seen
against the wall (negative after image). Each image should have a
central gap corresponding to the dot which indicates the visual
direction of the fovea. It is the position of this gap or space in the
image which is important.
TIP: The distance between the gaps is determined by the angle of
anomaly and not the angle of deviation; gaps show directional value
of foveas.
IMPORTANT: If ARC is revealed on this test, it is well established.
It is possible to show NRC on this test and ARC on other tests
because the after image test makes no attempt to produce normal
conditions of seeing and is more likely to reveal the original (normal)
correspondence.
Disadvantages
a. Difficult to use with uncooperative patients because of the
necessity of accurate observations.
b. Does not necessarily diagnose what the patient is doing under
every day conditions.
c. Complete suppression of one image may occur.
E. Major Amblyoscope (Synoptophore)
Patient is asked to move the arm of the troposcope before the nonfixing eye until they see the images superimposed or until they see
them cross from side to side. The subjective angle at which this
occurs is compared to the objective angle of deviation. The angle of
anomaly should not be less than 8 prism diopters because it can be
confused with:
1. Suppression - Small area of dense central suppression simulating
an angle of an anomaly.
2. Accommodation - angle of deviation may vary on dissociation.
3. Small degree of incomitance - apparent angle of anomaly as
patient changes fixation from one eye to the other.
IMPORTANT: Degree to which ARC is established can be
assessed using different size 1° and 2° slides.
Each eye is covered in turn and the patient is asked to note any
movement of the fixation object. The object should move to a
degree corresponding to the angle of deviation if NRC is present. If
no movement is observed, suspect ARC.
III. OBJECTIVE TESTS
Press-on prisms are a useful tool to assess patient response to
surgery.
When patients cannot reliably respond to sensory tests, the prism
adaptation test (PAT) may help to prognosticate surgical outcome.
Press-on prisms are given in an amount to slightly overcorrect the
deviation and are placed completely in front of the preferred eye in
children, non-preferred eye in adult, or equally in front of both eyes.
If after one to two weeks wearing the prism the patient returns to his
original deviation, this informs the surgeon to be more aggressive in
the surgical procedure.
IV. TREATMENT OF ARC PATIENTS
A. General Management
Occlusion to eliminate amblyopia or alternate occlusion to prevent
reinforcement of ARC which is a binocular phenomenon before
surgical management.
B. Cosmetically Acceptable Patients
If patient has a small angle deviation (10-20Δ), VA is fairly equal,
gross fusion and gross stereopsis, no treatment is given in an
attempt to break ARC. In fact, some authorities advocate
reinforcing and firmly establishing the ARC in order to give a more
refined and stable form of binocular vision. The method of
treatment used in firmly establishing the ARC is mainly with the
amblyoscope. Have the arms set at the patient’s subjective angle
and increase divergence and convergence amplitudes from the
subjective angle.
IMPORTANT: The majority of patients in this group are given no
treatment and are just observed at 6 month intervals to check if ARC
pattern is firmly maintained.
C. Orthoptic Management
1. Occlusion - to equalize VA and then alternate occlusion continued
preoperatively to prevent reinforcement of ARC.
2. If orthoptic treatment is considered, the orthoptic methods
performed on the Synoptophore are as follows:
a.
Bi-retinal kinetic stimulation
The major amblyoscope tubes are locked at the patient's
objective angle and the patient is instructed to fix straight
ahead into the distance through the pictures. The tubes are
then moved back and forth sideways to stimulate retinal
corresponding points. The patient must not watch the
pictures.
b. Massage of the macular area
The major amblyoscope tubes are set at the patient's objective
angle and the patient is instructed to fix steadily on the target
before the fixing eye. The other tube is moved from side to
side over the objective angle while the patient tries to see that
target crossing backwards and forwards over the fixation
object.
c. Mayou Technique
A foveal target is presented to the non-dominant eye and
boxes of varying size to the fixing eye. Starting with the largest
box, the patient is asked to move the tube from side to side
over the target. This may cover a large area at first, the area
gradually decreasing.
When the images can be seen
superimposed, smaller boxes are used until foveal targets can
be superimposed without movement.
d. Flashing
The major amblyoscope tubes are locked at the objective angle
and large, bright targets are used. The illumination is increased
to maximum before the non-dominant eye. Alternate flashing
is then carried out and the patient attempts to see the images
come closer and eventually superimpose.
All exercises are objective and can be dull for the patient. Many
patients may eventually give correct answers merely because they
are bored.
D. Surgery
Aim is placing the visual axis parallel in order to facilitate constant
bifoveal stimulation. Since patients with ARC have an intense
motivation to return to their anomalous angle, the surgery considered
is usually more aggressive than in patients with NRC.